Process for stabilizing molten polyethylene terephthalate



PROCESS FOR STABHJIZING MOLTEN POLY- I EIHYLENE TEREPHTHALATE EmmetteFarr Izard, Kenmore, and Cyrus Efrem Sroog, Grand Island, N. Y.,assignors to E. I.- du Pont de Nemours & Company, Wilmington, Del., acorporation of Delaware 7 No Drawing. Application September 30, 1953Serial No. 383,420

2 Claims. (Cl. 26075) This invention relates to a process of stabilizingthe molecular weight of a polymer in molten form, and, moreparticularly, to a process of stabilizing the molecular weight of amolten polyethylene terephthalate.

The production of the novel class of filmand fiberforming, linearpolyesters of terephthalic acid and a glycol of the series HO(OH ),,OHwhere n is an integer from 2 to 10 inclusive, is fully disclosed inUnited States Patent No. 2,465,319 to Whinfield and Dickson. acommercial standpoint, one of the most attractive polymers of this classis polyethylene terephthalate; and the most promising process for itsproduction comprises carrying out an ester interchange between ethyleneglycol and dimethyl terephthalate to form bis-Z-hydroxyethylterephthalate monomer which is polymerized to polyethylene terephthalateunder reduced pressure and at elevated temperatures.

In large scale or commercial production of polyethylene terephthalatefor extrusion into films or filaments, it is cosity begins to degrade,i. e., fall below the original Patented Dec. 23, 1958 usually resultsin-the formation of film having excessively non-uniform caliper; and thephysical properties of the film ultimately produced therefrom'will notbe equal in all respects to film produced from higher molecularweightpolymer. Y t a An object of the present invention, therefore, videa process of stabilizing the polymerization step and the stepof meteringthe molten polymer into extrusion apparatus. Other objects will beapparent from the description of the invention presented hereinafter. t.'1 The above objects are accomplished in accordance with the presentinvention by maintaining the molten undegraded polyethyleneterephthalate of the desired intrinsic C.

tating the molten polymen By undegraded will be undegraded, but it'mustbe emphasizedthat molecular weight of the of polymerization of thepolyester and may be defined as limit as C- approaches 0, wherein (n),.is the viscosity of a dilute phenol-tetrachloroethane solution (60-40)oftthe polyester divided by the. viscosity of the phenol-tetrathe sameunits atthe present-invention may also be applied to various modifiedpolyethylene terephthalates, that is, modified with small esters, fromthe group consistpolyethylene terephthalates and to facilitate agitationthereof, the temperature'must be-atleast about' 26'5'C.' and may be ashigh as about 280 C. Preferably, the temperature is within the-rangefrom275 C. to 280 C.

The pressure range, under which the molten polymer mass =mustbemaintained in order to insure molecular temperature range 4 out atatmospheric pressure and at a between 140-220 C., and preferably between150-200 C. Methanol is continuously withdrawn from the reactor duringester interchange, and

veniently carried 5 termination of evolution of methanol indicatessubstantial weight stability, is quite critical. If-the pressure on thecompletion of the ester interchange step. melt'is not maintained at asufficiently high-level,--i."e., The polymerization reaction is usuallyeffected in the at Ieast-l-S -mm., -further condensation mayoccur withliquid, i. e., melt, phase. In the liquid phase, the a consequentincrease in intrinsic viscosity. This is reaction must be carried out atreduced pressure, normalparticularly 'true where the polymerization isstopped ly in the vicinity of 0.05-5.0 millimeters of mercury.considerably short of -a maximum value, as in the This reduced pressureis necessary to remove the free preparationof-polymer for conversioninto filaments. In ethylene glycol which emerges from the polymer as ageneral, further polymerization will not occur if the result of thecondensation reaction. If reduced pressure pressure isat least '15millimeters of mercury. On the is not used, all the ethylene glycol willnot be removed; other hand, the maintenance of pressures substantiallyand it will not be possible to form a fiber or film-forming above 40mm.of mercury results'in more rapidreduction polymeric material, butrather, a .low molecular weight in molecular weight. Therefore,particularly with large polymer, too brittle for fibers or films. Atemperature batches (502,000'pounds) of molten polymer, the presbetweenabout 230 to about 290 C., and preferably sure should be maintainedwithin the range of from between about 260'to about 275 C., should'bemaina'bout l5-mrn. to about 40mm. of mercury to insure subtained duringthe polymerization step. Normally, a stantial molecular weight.stability. polymerization catalyst is employed, for example, lithargeAnothercritical aspect'ofthe present invention is that orantimony'trioxide. of maintaining an oxygen-free atmosphere above theThe following specific examples further illustrate the surface of themolten polymer mass during the hold-up principles and practice of thisinvention: period-(period duringwhichconstant molecular weight is Ineach of the examples, polyethylene terephthalate maintained). This isaccomplished by bleeding an inert was prepared by following the generalprocedure above gas,.e. g., nitrogen or carbon dioxide, into theatmosphere outlined; After polymerization was carried out (in a abovethe molten polymer. Theinert gas is metered into 50-pound capacityautoclave provided with a stirrer) to the system under very lowpressure, e. g., through a rethe desired intrinsic viscosity, e. g.,about 0.63, the temducingvalveand then through acapillary system. Theperature was adjusted to about'275" C. and, with conintroductionof theinert gas into the atmosphere above stant stirring, the pressure wasadjusted to the various the :molten polymer acts as, an oxygenscavenger, thus values specified in the following table 'for a period ofensuring 'the .maintenanceof a substantially oxygen-free eight hours. Asteady stream of nitrogen gas was bled atmosphere above the moltenpolymer. Stabilization into the autoclave above the level of the moltenpolymer. eannotbeaccomplished without maintaining a substantial- Thenitrogen was introduced into the system through :1 1y oxygen-freeatmosphere above the molten polymer. capillary system via a-reducingvalve. Intrinsic viscosity Furthermore, the inert, substantiallyoxygen-free atmosmeasurements were made at hourly intervals with thephere-must be relatively quiescent. In actual operations, results shownin Table I.

I Table 1 Polymerization Intrinsic viscosity, hours after adjustmentExample Catalyst Combination, percent 1 Conditions, Adjusted I 0]? (1m.) to, mm.

- 0\1\2l3\4\5 676 1 0.0 0859% LlH, 0.035% Zr1('OAc) 0,03 275 02-05 10.0. 63 0.65 0.66 0. 69 0.71 0.74 0.75 0.71 0.71 2 .oo og z 'rm, 0.035 znoAc ,,1 0,03 275-270/01-03 20 0. 04 0.65 0.63 0. 6a 0. 04 0.64 0.63 0.63 0. 03 8 orgig zg'mfi, 0.035% 'Zn(OA'c) 003 275 025 40 0.63 0.64 0.640.64 0.64 0.64 0.63 0. 63 0.03 4--- 00 057 1111, 0.035% Zn(0Ac):, 0.0327810.2 0.64 0.62 0. 63 0.63 0. 63'. 0. 61 0.50 0. 5s 0. 57 5 00 02 71411, 0.035%-.Zn(OAc) 0.03 275102-05 75 0.62 0.65 0.63 0.63 0.60 0.58 0.57 0.55 0.55 6 onots a 'mrt, 0.035% Zn(OAc) 0.03 276277/0.20.9 100 0. 610. 57 0. 57 0.57 0.56 0. 0. 55 0. 55 0.54

Emm t-0.35% tricresyl phosphate. r 7 0%)878 LiH, 0.035% Zn(OAc) 003 2770507 100 0.66 0.63 0.60 0. 59 0. 58 0.58 0. 57 0.56 0.56 00 0;575:113,0.035% Zn(OAc)z, 0.03 270-280/01 200 0.63 0.61 0.50 0.57 0.57 0.56 0.550. 54 0.53 orglghg mfi, 0.035% zntoaem 0.03 277278/0.1 760 0.63 0. 50 am0. 59 0.58 0.56 0.56 0. 56 0.55

1 Wt. percent of polymer. Zinc acetate dihydratc. it has been shown thatan appreciable increase, i. e., Example 1 illustrates adjusting thepressure in the increase over a relatively quiescent state, in thevelocity autoclave, after an intrinsic viscosity of 0.63 was reached, ofthe inert gas, e. g., nitrogen, over the surfac of the to 10 millimetersof mercury. Subsequent measurements molten polymer (even under thetemperature and presof intrinsic viscosity of this batch, the batchbeing consure conditions recited hereinbefore) leads to efiectingstantly stirred, indicate that the intrinsic viscosity infurtherpolymerization of the molten polymer instead of creases. This isprobably because the polymerization re- .Sflbstantial Stabilization ofthe molecular weight. action tends to continue at a pressure of 10millimeters, In the preparation of bis-Z-hydroxyethyl ter hth l talthough the polymerization reaction had been carried which issubsequently polymerized to form polyethylene Out under a pressureWithin the range 0.2-0.5 millimeter. terephthalate, an ester interchangeis carried out between On he Other hand, adlUST-mmt of ihe Pressure 9ethylene glycol and dimethyl terephthalate. Normally, millimetersresults in substantial stabilization at the onan ester interchangecatalyst is employed, for example, giflal intrinsic Viscositylitharge,combinations of lithium hydride and zinc acetate, The process of thepresent invention is highly useful combinations of magnesium acetate andcadmium and. advantageous in a continuous process for thepreparastearate. This initial condensation reaction may be con- 15 tionof a polyethylene terephthalate in the form of films 2000 lbs. of moltenpolymer. Molten polymer from these two sources may then be blended insuitable mixing equipment and thereafter held in a large vessel prior tobeing metered into extrusion equipment.

mer), the blending vessel acted as a hold-up vessel wherein moltenpolymer was maintained at substantially contions of about six hours;that is, the polymer in the blender was changed about every six hours.The polymer from the continuous unit was being continuously inured everyhour, the viscosity was always within the range from 0.590.61.

We claim:

1. A process for a polyethylene terephthalate polymer 1n molten formwhich comprises maintaining an undegraded polyethylene terephthalate ata'ternperature of from about 265 to about 280 C., and under a reducedpressure within the polymer, and continuously stirring the melt.

2. The process of claim 1 wherein the temperature is within the range of275-280 C. and an atmosphere essentially composed of nitrogen ismaintained above the surface of the molten polymer.

References Cited in the file of this patent UNITED STATES PATENTSEdwards et al. Apr. 11, 1950 OTHER REFERENCES Hardy: I. S. C, I. 67,pages 426-432, November 1948,

UNITED STATES PATENT OFFICE Certificate of Correction Patent No.2,865,892 December 23, 1958 Emmette Farr Izard et a1. It is herebycertified that error appears in numbered at t the printed specificationof the above en requiring correction and that the said Lett as correctedbelow.

ers Patent should read H columns 3 and 4, Tabl -HO (CH2) nOH-;respectively, for th opposite Example 1 and Example 6, (OAc) read-ZN(OAc) June 1959 thereof, at portlon reading Zn S1gned and sealed thiAttest KARL H. AXLINE, ROBERT C. WATSON, Attestz'ng Oficer.ommz'ssz'oner of Patents,

1. A PROCESS FOR STABILIZING THE MOLECULAR WEIGHT OF A POLYETHYLENETEREPHTHALATE POLYMER IN MOLTEN FORM WHICH COMPRISES MAINTAINING ANUNDEGRADED POLYETHYLENE TEREPHTHALATE AT A TEMPERATURE OF FROM ABOUT265* TO ABOUT 280*C., AND UNDER A REDUCED PRESSURE WITHIN THE RANGE OFFROM 15 TO 40 MILLIMETERS OF MERCURY, MAINTAINING AN INERT RELATIVELYQUIESCENT, ATMOSPHERE SUBSTANTIALLY FREE OF FREE OXYGEN ABOVE THESURFACE OF THE MOLTEN POLYMER, AND CONTINUOUSLY STIRRING THE MELT.